Report From the Red Planet: More than a year after their predicted demise, NASA's Mars rovers Spirit and Opportunity keep going, and going . . .

Get a taste of what awaits you in print from this compelling excerpt.

Surely the ghost of John Wesley Powell is out there somewhere, shouting a hurrah for his two new companions in the annals of heroic one-armed geology. When Powell led a three-boat expedition through the Grand Canyon more than a century ago, planets were but fuzzy dots in telescopes and space travel rarely even a dream. But the Civil War veteran, who lost half his right arm at the Battle of Shiloh, would grasp exactly what two NASA rovers and their earthbound handlers have done since January 2004, when the machines alighted on Mars to hunt for signs of ancient water.

In Powell's day geologists puzzled over how water sculpted the pink-hued Colorado Plateau into a canyon maze; their counterparts today wonder about water's role in carving landforms on red-hued Mars. Just as Powell's ragtag company defied expectations in 1869 by surviving nearly a hundred days of savage rapids, the rovers, expected to conk out well before the end of last year, were going strong months later. Powell was a field geologist, cracking rocks and taking notes with his single arm. So too each rover uses its three-jointed arm to wield equipment including a camera and a tool for grinding into Mars rocks.

Rock-breaking brings up the essential reason to muse over these robots and old-timer Powell: ground truth. This is not a casual term among geologists. It evokes the dust-on-boots conclusions that arise from personally grabbing samples of rock, walking formations, and exposing fresh stone. Generations of geologists have placed their literal, bedrock faith in ground truth.

Ground truth gives scientists the confidence to interpret wide stretches of geography seen from afar. Powell climbed mile-high cliffs while scrutinizing their colorfully layered limestone, sandstone, marble, chert, volcanic rock, and gneiss. Once on top he could gaze across tens of miles of canyonland, sketch maps in his notebook, and knowingly label distant strata from their colors and continuities. Similarly, thanks to reports from the six-wheeled rovers exploring two small patches of Mars, researchers can make sense of the sometimes ambiguous images from satellites orbiting high above.

Take the sinuous channels visible on Mars's parched countenance. They so resemble huge, dry riverbeds that flowing water in some long-ago epoch seemed a very good bet. But proof was lacking. Some scientists argued that frigid surges of liquid carbon dioxide might have carved the channels. Others said Mars was never warm enough for more than the briefest pulses of water because it seems to lack the extensive limestone and other carbonate minerals that a warm, carbon-dioxide-rich atmosphere would have left behind. "I spent my career describing water on Mars," says Mike Carr, a geologist at the U.S. Geological Survey in Menlo Park, California. "But we just couldn't find the carbonate. I was starting to tell people my whole career maybe was ashes."

No longer. The rovers, Spirit and Opportunity, have found direct and convincing evidence that water sometimes sloshed across Mars, almost certainly during the planet's earliest epoch well over three billion years ago. Perhaps only puddles, streams, and flash floods came and went, drying repeatedly. But it's looking more and more likely that liquid water was once abundant, though short-lived. Even oceans, however ephemeral, remain a possibility, and on a planet once warmer and wet, life—past or present—is a tantalizing prospect.